Whether you're a visiting tourist or seasoned resident of Japan, finding a decent meal if you're a vegetarian or vegan can be a positively fiendish task.
Though animal products don't often occupy the plate in the same proportions as they do in the typical meat-and-two-veg Western diet, they are ubiquitous in Japanese cooking, and often turn up in unexpected or unseen places. For example, dashi, the stock that is the foundation of so much Japanese cuisine, is most commonly made with a mix of konbu seaweed (vegetarian) and katsuobushi (skipjack tuna; definitely not vegetarian).
Websites such as The Happy Cow do exist to provide listings of local vegetarian and vegan restaurants, but the most common advice to those seeking to maintain their diets while in Japan still seems to be based on temple stays, where you can dine on shōjin ryōri with Buddhist monks, or confining yourself to a diet of umeboshi- (pickled plum) filled onigiri rice balls from convenience stores. As a temporary fix this might work, but yields food that is at best expensive and inconvenient, and at worst bland and uninspiring.